But it's the caption accompanying the post — "Sir Carter and Rumi 1 month today" — that sent Twitter into a frenzy.

No one really uses last names when referring to Beyonce (Knowles) or Jay-Z (Carter), but we can only assume the twins do indeed have surnames. (Their older sister's Instagram profile is called Blue Ivy Carter, after all). So is the billion-dollar music couple's newborn son named "Sir Carter Carter," as Beyonce's post would suggest? Or is it just Sir?